I strongly disagree with anyone who says you should be riding a 48, or 50 cm frame. Assuming that you have a bike now, measure your preferred seat height. (seat top to the crank center to the top of the seat), and try out a 42 cm bike with your preferred seat height. On a mountain bike, if a bike were 48-50 cm (19-20"), that would be for someone close to six feet tall.